Children with specific language impairment (SLI) consistently show poor nonword repetition (NWR) performance. However, the reason for these difficulties remains a matter of intensive debate. Nonword repetition is a complex psycholinguistic task that heavily relies upon phonological segmentation and phonological knowledge, and even lexical knowledge. This study aims at investigating various linguistic factors that can be at the root of difficulties in children with Specific Language Impairment when repeating nonwords, with the goal of achieving a better understanding of the linguistic processes supporting nonword processing. Linguistic complexity was assessed by manipulating lexicality, syllabic complexity, and perceptual difficulty in NWR tasks. Fifteen children with Specific Language Impairment, 15 typically developing controls matched on both age and performance IQ, and 15 typically developing children matched on lexical knowledge participated in this study. Children with Specific Language Impairment performed overall more poorly than age- and IQ-matched children and lexical age–matched children. Importantly, children with Specific Language Impairment showed lower lexicality and syllabic complexity effects in their NWR performances. These results are compatible with difficulties to retrieve lexical and sublexical phonological knowledge in the context of NWR tasks.